Upon world load, animals often teleport to just outside their completely fenced-in area. I have seen this happen with cows, sheep, pigs, and chicken. I have not tried to see if this also happens upon general chunk reload. I will try to post screenshots soon. This happens on about 80% of world loads. The animals do not just appear to leave the fence because they do wander off and have to be lured back in.

In the world , leave the shack through the front iron door and turn left. The animals on your left are likely to have left the fence.